Blow molding machines conventionally operate by introducing a parison, i.e. a tube of hot plastic, between open sections of a mold. Closing of the mold then clamps the parison and allows air to be blown into the parison such that it assumes the shape of the mold. After suitable cooling has taken place, the mold sections are opened to allow ejection of the molded part.
Blow molded parts such as containers have conventionally included paper labels that are glued thereto after the blow molding to identify the contents of the container to the consumer. One problem is that such paper labels can become wrinkled if dampened and can also become detached from the container if a waterproof glue is not used. Also, paper labels require a separate labeling step after the molding which thus adds to the cost of the container.
In-mold labeling of plastic blow molded containers was initially developed in the early 1960's. This in-mold labeling is performed by initially inserting a label within the mold prior to the introduction of the parison and by then closing sections of the mold in preparation for the blowing operation. The subsequent blowing operation forms the parison around the label to the shape of the mold and provides a permanent bond which is incapable of being broken by moisture or otherwise. Also, such in-mold labeling provides a smooth transition between the label and the adjacent surface of the molded plastic part and further provides additional strength since the label cooperates with the plastic in resisting deformation.
U.S. Pat. No. 3,108,850 Brandt discloses a relatively early plastic container made by blow molding with an in-mold labeling operation. This container is blow molded from a polyethylene parison to which a polyethylene label is applied by the in-mold labeling operation performed during the blow molding process. The label is disclosed as being secured to the mold wall by moisture that largely eliminates the intervening air layer such that an unbalanced air pressure provides the securement. Also, the surface of the label that is engaged by the blown container is disclosed as having ridges, minute points or pyramids which are alleged to provide enhanced securement to the container when both the label and the container are made of polyethylene.
In U.S. Pat. No. 3,207,822 Makowski, cylindrical polyethylene containers are blow molded with an in-mold labeling operation wherein a polyethylene label is disclosed as being secured to the mold wall by a vacuum, an adhesive, static electricity, or by applying a moisturing and air excluding agent such as water. The cylindrical mold wall is also disclosed as having a grooved, waffled, or otherwise roughened surface for enhancing label securement.
Blow molded containers have also more recently been made from polyethylene terephthalate such as disclosed by U.S. Pat. Nos.: 3,733,309 Wyeth et al; 3,802,275 Corsover; and 3,948,404 Collins et al. In the Corsover patent, the polyethylene terephthalate container is made with an amorphous construction while the Wyeth et al and Collins et al disclose cylindrical polyethylene terephthalate containers that are biaxially oriented to provide increased mechanical strength both axially and in a circumferential direction.